This invention relates to a device and a method for determining a runway state, as well as to an aircraft landing and/or takeoff assistance system and method, and to aircraft equipped with such devices and systems.
During the landing and takeoff, and more generally the taxiing phases of an airplane, knowledge of the surface state of the runway is of major importance. Prediction of the braking performance of the airplane indeed depends on this knowledge. It thus is possible:                to best estimate the distance necessary for stopping the airplane in a concern for safety,        not to overestimate this stopping distance necessary for bringing the airplane to a standstill and therefore not to overly penalize utilization operations of the runway and the airplane.        
Now, braking performances of an airplane on a so-called contaminated runway are very difficult to predict because of the difficulty in knowing reliably and precisely the contribution of the runway state to the deceleration of the airplane, in particular in terms of adhesion and of projection and displacement drags in the case of a deep contaminant. The contaminants can be any element happening to be deposited on the “original” runway, as for example rubber deposited during previous landings, oil, rainwater forming a more or less uniform layer on the runway, snow, ice, etc.
Knowledge of such a contribution of the runway can seem beneficial for improving landing systems such as the one described, for example, in the document FR-2897593.
This knowledge also can prove important for increasing the takeoff security of airplanes, the latter having to estimate, for example, the runway point of no return no longer permitting a completely safe emergency braking on the remaining runway portion.
Initial solutions for estimating the runway state already have been set up, but measurements of the runway adhesion today are very difficult, ineffective, unreliable and hard to transpose from the context of the measurement means used to that of an airplane taxiing on the actual runway.
There is known in particular the measurement of adhesion via traction engines or “mu-meters,” for example towed vehicles or special cars, that provide results that are disparate, potentially inconsistent among themselves, non-representative for an airplane because of different scales of phenomena such as the stresses and performance of the tires, and which moreover necessitate a closing of the runway during the measurements.
In practice there also is recourse to visual and manual inspection of the runway by control which then provides a type and a depth of contaminant which are or are not compatible with the performance calculation means of the airplanes. This approach, however, provides only an indication highly dependent on the place where this inspection was conducted.
Also, there are known “Reported Braking Actions” which in fact are the experience of the pilot of the previous airplane concerning his braking performances with a division into four simple levels: good/medium/poor/nil (in practice indicated by the following English terms: “good”/“medium”/“poor”/“nil”), from which it is possible to manually inform the airplane on approach for landing. But this solution is subjective, depends on the airplane and takes into account contributions other than the braking of the wheels (the pilot being unable to identify the precise part of the various braking means of his airplane: aerodynamics, engine thrust or counter-thrust and braked wheels).
On the contrary, this invention relates to a solution for estimating a runway state that is more objective and representative of the behavior of airplanes.
In this sphere, analysis solutions applicable later on the ground already have been developed for estimating a posteriori the state of the runway at the time of an incident or an accident in service, or for validating trial flights in “real time.”
These solutions generally rely on measurements of the deceleration of the airplane during landing. Then on the ground, delayed treatments are performed in order to estimate the adhesion of the runway on the basis of this measured deceleration, subtracting therefrom in particular aerodynamic, engine and contaminant components or contributions deriving from models based on other measurements performed on the airplane or outside.
These treatments performed take into account the type of airplane involved, since the measurement of deceleration alone does not allow an easy utilization by another airplane.
Moreover, these treatments are long, manual and not compatible with an intensive operation of an airport where an estimation of the state of the runway is required in a brief period before the following airplane in turn executes a phase of taxiing on the runway, either for landing or for taking off.
Furthermore, there is known from the document US 2006/243857 a method and a device for estimating characteristics relating to a landing runway. A real-time treatment is carried out, during which various airplane or external parameters are acquired and recorded. From these recorded parameters, an estimation of the deceleration due solely to braking is performed on the basis in particular of the deceleration Ax of the airplane, the engine thrust Areverse thrust and the aerodynamic drags Adrag. A friction profile “μ” then is established in order to determine whether or not the airplane is at braking limit, and to let the pilot know accordingly.
These information items, however, simply cannot be used in a time period satisfactory for informing the airplanes in approach.